Elemental phosphorus is widely used as a dopant in the manufacture of semiconductor devices, and as an ingredient in III-V compound semiconductor materials, e.g., InP, GaAlP, GaAlAsP. It is a principle precursor material for Molecular Beam Epitaxy (MBE). In semiconductor processing in general, phosphorus is employed in the form of a gas compound, typically PH.sub.3 or PCl.sub.3. These gases are hazardous, and the trend in semiconductor processing technology is toward in situ generation of hazardous gases, including arsine, phosphine, and the halogenated forms of phosphorus and arsenic. For in situ generation, the precursors are typically the elements arsenic and phosphorus which, while toxic themselves, pose less risk in handling and storing than the gas compounds.
Elemental phosphorus is known to occur in three allotropic forms, white, red and black. White phosphorus is a highly toxic substance by all routes of exposure. It ignites spontaneously and burns forming an acrid white vapor that is dangerous to the eyes and respiratory tract. It autoignites at 29.degree. C., and under some circumstances at even lower temperatures, and therefore must be stored and handled with great care.
Red phosphorus is less reactive, and less toxic, than white phosphorus. However, red phosphorus typically contains white phosphorus as an impurity, and itself autoignites at elevated temperatures. It also reacts readily with water at elevated temperatures to form toxic phosphine. Consequently there are strict handling and storage regulations for red phosphorus which increase the cost of this material and limit use of red phosphorus as a commercial reagent.
Black phosphorus resembles graphite in texture and has an uncommon layered crystal structure, with layers of one atom bonded to three atoms. The important technological feature of black phosphorus is that it is relatively stable and inert, and difficult to ignite in air. Thus it can be stored and handled with low risk. However, the known techniques for synthesizing black phosphorus are based on the conversion of white phosphorus at very high pressure. A process that uses white phosphorus at very high pressure is itself extremely hazardous. If a process was available for safely producing black phosphorus it could become an important precursor material in semiconductor technology, thus reducing a source of risk and concern to both technologists and environmentalists.